Reconstructing and Dating the Past

1. Reconstructing and Dating the Past Preservation and Dating Methods

2. Preservation Not everything made by past peoples survive and make it into the archaeological record. Softer more perishable subsistence, such as cloth, often disintegrates very quickly, especially in soils that are particularly acidic. Best conditions for preservation: Very wet Very cold Worst conditions (those that change): Hot ? cold Wet? dry Tropics

6. Dating Methods Relative dating techniques�refers simply to evaluating the age of one item of data relative to other items. EX: determining if artifact A is older than artifact B. Absolute dating techniques� provides an absolute date in the form of a calendar date.

7. Relative Dating Seriation�patterns of human behavior change continually, and as behavior changes, so do its material products. We observe how changes through time in design and style are after familiar objects in our own society. The artifacts and features of past societies also exhibit changes through time, and by observing and studying their attributes, archaeologists can usually discover the trends.

8. Sequence Comparison If seriation cannot be used for the artifacts being studied, the archaeologist has another recourse. Sequence comparison-- If other well-documents artifact sequence exists in the geographical area being investigated, the artifact classes in question may be compared to those already defined form nearby sites and placed into a temporal order corresponding to those already established.

9. Stratigraphy Stratigraphy refers to the archaeological interpretation of the significance of stratification. The age of archaeological materials can sometimes be assessed by their association with geologic deposits or formations. Often these assessments are relative, as in cases based on superposition, where materials in lower strata were deposited earlier that those in higher strata. As long as that context� and, therefore, the temporal order of a stratified deposit is clear, the archaeologists can use stratigraphy to determine the relative age of the deposit of artifacts and other materials in the deposit.

11. Absolute dating techniques Chronometric, or time-measured techniques Radiometric method Potassium/Argon (40K40Ar) Optically stimulated luminescence (OSL) and Thermoluminescence (TL) Electron Spin Resonance (ESR) Fission track dating

12. Carbon 14 This method was developed in the late 1940s by Willard F. Libby, who was measuring the amount of 14C detected by Geiger counters, which is used to measure the rate of decay emissions from a sample. Half-life 5730 � 40 Several age-determination techniques exploit the principle of radioactive decay, the transformation of unstable radioactive isotopes into stable elements.

13. Radiocarbon dating is the most important radiometric technique for archaeologists. Carbon dioxide enters plants through photosynthesis, and the plants are in turn eaten by animals. Thus all living things constantly take in both ordinary carbon (12C) and radioactive carbon (14C) throughout their lifetimes. The proportion of 14C to 12C in an organism remains constant until its death.

14. At that point, no further 14C is taken in, and the amount of radioactive carbon present at that time begins to decrease through radioactive decay. Thus, measurement of the amount of 14C still present (and emitting radiation) in plant and animal remains enables the determination of the amount of time elapsed since death.

16. Potassium/Argon (40K/40Ar) Based on the radioactive decay of a rare isotope of potassium (40K )to form argon (40Ar )gas. The half-live of 40K is 1.31 billion years, but the method can be used to date materials as recent as 100,000 years old. The technique is used principally to determine ages for geological formations that contain potassium. The 40K/40Ar technique has been particularly helpful in dating geological formations associated with the remains of fossil hominids and lower Paleolithic tools.

17. Optically stimulated luminescence (OSL) and Thermoluminescence (TL) These are related dating techniques which attempt to measure the amount of energy trapped in sediments (OSL) or heated clay and stones (TL) by reheating the material and then measuring the amount of energy releases in the process. The common principle uniting the approaches is the exposure to radiation causes electrons to separate from atoms. Some of these electrons are caught in what are called traps, or defects in the material being heated. This process continues until the traps are full�saturation.

18. By exposing the �traps� to radiation in the lab, the traps can be opened and the energy that is emitted in the form of light can be accurately measured. This can be measured and produces a dated based on the total amount of radiation received by the sample. However, OSL and TL can be problematic. The annual dose of radiation received by the sample must be accurately estimated. They change over time. Moisture history of the sample is also crucial, because water attenuates (reduces the strength or value) radiation effects. This process destroys the sample.

19. Electron Spin Resonance (ESR) A technique best suited for the analysis of tooth enamel, shells, and burnt stone, it uses a spectrometer to measure the amount of energy released from an object when bombarded with microwaves. Has an effective range between a few thousand to 500,000 ya. This method used microwaves to bombard the sample and then used an ESR spectrometer to measure the trapped electrons.

20. Fission track dating This method functions differently from the 3 just mentioned. When atoms of 238U (uranium 238) fission, they do so at a constant rate (8.2 � 1015 years). In the process of fissioning, the nucleus bursts apart into two parts that are violently repelled from on another. These atomic bodies create fission tracks in crystalline material and obsidian. By counting the frequency of fission tracks and comparing these to the know rate of fissioning, the age of the sample can be calculated.